The invention relates generally to electrical machines and, more particularly, to a control scheme for exciting an electrical machine with instantaneous non-sinusoidal current waveforms.
The usage of electrical machines in various industries has continued to become more prevalent in numerous industrial, commercial, and transportation industries over time. There has been tremendous progress and great achievements in the field of power electronics and control techniques for such electrical machines, resulting in increased energy savings and control flexibility. Providing for such achievements has been the continued progress in computer technology that has resulted from digital technology. Digital technology has lead to very significant reductions in the size and cost of computers, allowing them to successfully replace old, bulky, and relatively expensive mechanical systems.
While the capability of digitally enhanced control systems and computers has progressed, the structure of the electrical machines used with such control systems has, for the most part, remained unchanged. As shown in FIGS. 1A and 1B, for example, prior art electrical machines 6 used today across the board in many fields, especially for hybrid applications, are equipped with integral-slot distributed windings 8 that produce a fairly sinusoidal rotating field in the air gap when excited by AC currents. FIG. 1A illustrates a 24-slot, overlapping distributed arrangement of windings 8, while FIG. 1B illustrates a 12-slot, overlapping concentrated arrangement of windings 8 as such configurations are known in the art. These integral-slot distributed windings that are still in use are designed for the ideal sinusoidal wave forms based on earlier machines designed a century ago for use with 60 Hz mains supply. The inverters in existing electrical machines that are used to feed these windings with sinusoidal current wave forms are thus designed using pulse width modulation (PWM) techniques. These PWM techniques use very high frequency carrier signals, resulting in high switching losses in the inverter devices, as well as significant reduction in the life of the stator insulation system of the electrical machine.
With specific reference to the use of electrical machines for hybrid applications, which have tight packaging constraints, the need to obtain high power density machines necessitates running these machines at high speeds. This requires high fundamental excitation frequencies, which generates high frequency harmonics resulting in large eddy current losses in the stator laminations. In order to reduce these losses, designers have to use thin laminations, which can be prohibitively expensive.
In order to overcome the drawbacks associated with conventional electrical machine designs, designs have been developed with alternate winding configurations. Fractional-slot concentrated windings (sometimes referred to as tooth windings), for example, have been developed as an alternative configuration (see FIGS. 3A and 3B, for example). Such windings are simpler, easier to manufacture, less expensive, and help improve the machine power density. However, tooth windings introduce increased levels of space harmonics that produce non-sinusoidal rotating fields in the machine air gap. These non-sinusoidal fields generate losses in both the stator and the rotor and hence reduce the machine efficiency.
Therefore, it would be desirable to design an electrical machine that can directly accept non sinusoidal current wave forms while maintaining high power density and high efficiency. It is further desired that a control scheme be provided for controlling the machines that suppresses the effect of the additional harmonic components typically associated with tooth windings.